The present invention relates to a fuel cell and relates more particularly to a fuel cell which has been improved to prevent a corrosion of a gas manifold due to phosphoric acid.
A fuel cell is used for generating electric energy based on an electrochemical reaction of hydrogen obtained by reforming a hydrocarbon fuel such as natural gas or methane gas with air as an oxidizer, both supplied to a main body of the fuel cell, in the presence of an electrolyte. The fuel cell has a laminated structure (a cell stacked structure) by a lamination of a plurality of single cells having the above-described power generating function.
FIG. 1 is a breakdown perspective diagram for showing the structure of a cell stack of a fuel cell which has been used conventionally. A single cell 1 of a fuel cell main body is structured by a fuel pole 3 for supplying hydrogen disposed at one side of the plane of a matrix layer 2 holding an electrolyte in the direction of an arrow A in the diagram, an air pole 4 for supplying air disposed at the other side of the plane in the direction of an arrow B in the diagram, grooved electrode materials 5 and 6 laminated on the fuel pole 3 and the air pole 4 respectively, and a separator 7 laminated on one of the grooved electrode materials 5 and 6. Each time when the unit cell 1 is laminated to form a plurality of unit cells, a cooling panel 8 is inserted between each unit cell to structure one sub-stack 9, and the sub-stack 9 is laminated by a large number to structure a cell stack 10.
Further, a clamping panel 11 is installed at the top section and the bottom section of the cell stack 10 respectively. The cell stack 10 and the upper and lower clamping panels 11 are fastened by a tie rod 12 to be integrated as a cell stack unit 13.
Further, a pair of fuel gas manifolds 15a and 15b and a pair of air gas manifolds 16a and 16b are disposed on mutually opposite side planes of the cell stack unit 13 of the above-described structure so that a fuel gas and air flow in mutually orthogonal directions, as shown in FIG. 2.
Further, a gasket 18 is provided at each contact plane between the cell stack unit 13 and each of the gas manifolds 15a, 15b, 16a and 16b respectively to prevent in advance an occurrence of a problem of a reduction in the power generating efficiency due to a leakage of air or a fuel gas.
When fuel and air are supplied to the gas manifolds 15a and 16a respectively, a part of phosphoric acid impregnated in the matrix layer 2 and the grooved electrode materials 5 and 6 for structuring the cell stack 10 is diffused into the flow of the fuel gas and air respectively and is then exhausted to the outside of the cell stack (that is, into the gas manifolds) in the state of a phosphoric acid vapor.
However, since the temperature of the gas manifolds is slightly lower than the temperature of the cell stack, a part of the phosphoric acid vapor exhausted into the gas manifolds is condensed and adheres to the inner wall of the gas manifolds. When the fuel gas and air including phosphoric acid are brought into direct contact with the inner side of gas manifolds made of metal, the gas manifolds made of metal are corroded significantly in a high-temperature state, so that there is a risk that the gas manifolds soon have holes.
In order to eliminate the above-described drawback, a method of coating the inner plane of each gas manifold with a fluororesin as disclosed in the U.S. Pat. No. 4,950,563 has been used, as a method of protecting the gas manifolds from the corrosion by phosphoric acid.
However, the method of coating a fluororesin on the inner plane of the gas manifolds has the following problems and, therefore, it has been difficult to completely prevent the gas manifolds from being corroded by phosphoric acid.
According to the method of coating a fluororesin on the inner plane of a gas manifold, there has been a problem that phosphoric acid enters inside from pin holes and a problem that there occurs a failure in the adhesion of the resin coating on the inner surface of the gas manifold due to a repeated change in the temperature by the starting and stopping of the operation of the fuel cell and changes in the load since the coefficient of linear expansion of the resin coating is as large as about ten times that of the gas manifold, which results in a removal of the coating from the surface of the gas manifold.
Further, there is a problem that since a paint film formed by coating has a relatively small thickness, phosphoric acid can easily penetrate into the paint film, which results in a corrosion of the base material. Thus, this method lacks in reliability. Further, in order to improve the reliability of the coating, it is necessary to increase the thickness of the paint film, which requires a repetition of heating, coating and cooling processes many times. This requires a large processings time and a large number of processing. Moreover, since the coating process forms a part of a series of the manufacturing process of the gas manifold, these increases in the processing time and the number of processings have interfered to reduce the manufacturing period.
Furthermore, if the coating is broken and peeled off during the operation of the fuel cell, it is not difficult to detect this occurrence in a short time. This further has a problem of generating a corrosion of the gas manifolds and a failure of electric insulation.